An analog-to-digital (A/D) converter converts an input analog signal to an output digital signal that is an approximation of the input analog signal. The resolution of an A/D converter defines the accuracy of the approximation between the output digital signal and the input analog signal. In this regard, the closer the resemblance between the output digital signal and the input analog signal, the greater the resolution of the A/D converter. A/D converters may be designed with various architectures known in the art. In general, each A/D converter architecture can have specific characteristics making it suitable or unsuitable for a particular application.
One A/D conversion method is known as algorithmic A/D conversion. An algorithmic A/D converter may be referred to as a cyclic A/D converter, and such terms are used interchangeably herein. Algorithmic A/D converters operate in a manner similar to successive approximation. In successive approximation, an analog voltage is compared to a reference voltage over a number of cycles. As used herein, each cycle is referred to as an A/D converter stage.
In each stage, the analog voltage is compared to the reference voltage to determine a digital bit value. If the input voltage is greater than the reference voltage, the reference voltage is subtracted from the input voltage. The remaining voltage, referred to as either the remainder or residue, is input to the next stage for more accurate comparison. In each successive stage, the reference voltage is generally halved to increase accuracy of the comparison with the residue and determine the next less-significant bit. In algorithmic A/D conversion, a fixed set of reference voltages is used for each comparison. As with successive approximation, the analog voltage is compared with a reference voltage to determine a digital value. However, in successive stages the residue is generally doubled and again compared with the fixed reference voltages to increase accuracy and determine the next less significant bit.
Algorithmic A/D converters provide an architecture for performing A/D conversion that is efficient in terms of required hardware and power. However, algorithmic A/D converters often require digital calibration of analog errors, which reduces the range of the analog signal that can be accurately quantized to digital values. The ideal transfer function of an A/D converter is a straight line with unity slope where the digital output code is mapped perfectly onto the input signal range. The two common errors of a real implementation are the zero-order error and the first-order error. The zero-order error corresponds to the mapping of each code onto the signal range with the same offset to the ideal. The first-order error corresponds to a slope error of the straight-line mapping. The zero-order error and first-order error may also be referred to as offset error and gain error, respectively. To compensate for these errors, digital calibration is performed to correct the measured values. Uncorrected digital values are determined and stored. Subsequently, a calculation is performed using these stored values to determine corrected values.
For example, if every measured voltage is high by 10 mV, then the correction subtracts a digital value corresponding to 10 mV from all measurements. A zero volt input signal will result in a raw code corresponding to 10 mV, but after correction will result in a final code corresponding to zero. However the full scale output, for most A/D converters, is limited to (or saturates at) a digital value representing the full scale of the input range so that after correction a code corresponding to full scale minus 10 mV will result. All input signals within 10 mV of full scale will similarly saturate, and the net effect is that the output range goes from zero code to only code corresponding to full scale minus 10 mV. Digital calibration of analog errors in an A/D converter consumes part of the signal range to perform the correction in existing algorithmic A/D converters.
One or more embodiments of the disclosure may address one or more of the above issues.